Load Control Device for a Light-Emitting Diode Light Source

This application is a continuation of U.S. patent application Ser. No. 18/602,360, filed Mar. 12, 2024; which is a continuation of U.S. patent application Ser. No. 18/297,697, filed Apr. 10, 2023, now U.S. Pat. No. 11,979,955 issued May 7, 2024; which is a continuation of U.S. patent application Ser. No. 17/881,774, filed on Aug. 5, 2022, now U.S. Pat. No. 11,653,431, issued May 16, 2023; which is a continuation of U.S. patent application Ser. No. 17/001,050, filed on Aug. 24, 2020, now U.S. Pat. No. 11,412,593, issued Aug. 9, 2022; which is a continuation of U.S. patent application Ser. No. 16/601,845, filed on Oct. 15, 2019, now U.S. Pat. No. 10,757,773 issued Aug. 25, 2020; which is a continuation of U.S. patent application Ser. No. 16/378,134, filed on Apr. 8, 2019 now U.S. Pat. No. 10,448,473 issued Oct. 15, 2019; which is a continuation of U.S. patent application Ser. No. 15/953,812, filed on Apr. 16, 2018 now U.S. Pat. No. 10,257,897 issued Apr. 9, 2019; which is a continuation of U.S. patent application Ser. No. 15/717,123, filed on Sep. 27, 2017, now U.S. Pat. No. 9,949,330 issued Apr. 17, 2018; which is a continuation of U.S. patent application Ser. No. 15/460,973, filed Mar. 16, 2017, now U.S. Pat. No. 9,814,112 issued Nov. 7, 2017; which is a continuation of U.S. patent application Ser. No. 15/291,308, filed Oct. 12, 2016, now U.S. Pat. No. 9,635,726 issued Apr. 25, 2017; which is a continuation of U.S. patent application Ser. No. 14/796,278, filed Jul. 10, 2015, now U.S. Pat. No. 9,497,817 issued Nov. 15, 2016; which is a continuation of U.S. patent application Ser. No. 14/290,584, filed May 29, 2014, now U.S. Pat. No. 9,113,521 issued Aug. 18, 2015; which claims the benefit of U.S. Provisional Patent Application No. 61/828,337, filed May 29, 2013, the contents of which are hereby incorporated by reference in their entirety.

Light-emitting diode (LED) light sources are often used in place of or as replacements for conventional incandescent, fluorescent, or halogen lamps, and the like. LED light sources may comprise a plurality of light-emitting diodes mounted on a single structure and provided in a suitable housing. LED light sources are typically more efficient and provide longer operational lives as compared to incandescent, fluorescent, and halogen lamps. In order to illuminate properly, an LED driver control device (i.e., an LED driver) may be coupled between a power source (e.g., an alternating-current (AC) source) and the LED light source for regulating the power supplied to the LED light source. The LED driver may regulate either the voltage provided to the LED light source to a particular value, the current supplied to the LED light source to a specific peak current value, or may regulate both the current and voltage.

LED light sources may comprise a plurality of individual LEDs that may be arranged in a series and parallel relationship. In other words, a plurality of LEDs may be arranged in a series string and a number of series strings may be arranged in parallel to achieve the desired light output. For example, five LEDs in a first series string each with a forward bias of approximately three volts (V) and each consuming approximately one watt of power (at 350 mA through the string) consume about 5 W. A second string of a series of five LEDs connected in parallel across the first string will result in a power consumption of 10 W with each string drawing 350 mA. Thus, an LED driver would supply 700 mA to the two strings of LEDs, and since each string has five LEDs, the output voltage provided by the LED driver would be about 15 volts. Additional strings of LEDs can be placed in parallel for additional light output, however, the LED driver should be operable to provide the necessary current. Alternatively, more LEDs can be placed in series on each string, and as a result, the LED driver should also be operable to provide the necessary voltage (e.g., 18 volts for a series of six LEDs).

LED light sources are typically rated to be driven via one of two different control techniques: a current load control technique or a voltage load control technique. An LED light source that is rated for the current load control technique is also characterized by a rated current (e.g., 350 milliamps) to which the peak magnitude of the current through the LED light source should be regulated to ensure that the LED light source is illuminated to the appropriate intensity and color. In contrast, an LED light source that is rated for the voltage load control technique is characterized by a rated voltage (e.g., 15 volts) to which the voltage across the LED light source should be regulated to ensure proper operation of the LED light source. Typically, each string of LEDs in an LED light source rated for the voltage load control technique includes a current balance regulation element to ensure that each of the parallel legs has the same impedance so that the same current is drawn in each parallel string.

In addition, it is known that the light output of an LED light source can be dimmed. Different methods of dimming LEDs include a pulse-width modulation (PWM) technique and a constant current reduction (CCR) technique. Pulse-width modulation dimming can be used for LED light sources that are controlled in either a current or voltage load control mode. In pulse-width modulation dimming, a pulsed signal with a varying duty cycle is supplied to the LED light source. If an LED light source is being controlled using the current load control technique, the peak current supplied to the LED light source is kept constant during an on time of the duty cycle of the pulsed signal. However, as the duty cycle of the pulsed signal varies, the average current supplied to the LED light source also varies, thereby varying the intensity of the light output of the LED light source. If the LED light source is being controlled using the voltage load control technique, the voltage supplied to the LED light source is kept constant during the on time of the duty cycle of the pulsed signal in order to achieve the desired target voltage level, and the duty cycle of the load voltage is varied in order to adjust the intensity of the light output. Constant current reduction dimming is typically only used when an LED light source is being controlled using the current load control technique. In constant current reduction dimming, current is continuously provided to the LED light source, however, the DC magnitude of the current provided to the LED light source is varied to thus adjust the intensity of the light output.

However, an LED light source may become instable or exhibit undesirable characteristics when dimmed to a low intensity level or when dimmed to off (i.e., 0% intensity). For example, when dimmed to a low intensity level or off, an LED light source may flicker, may exhibit inconsistent brightness or color across the individual LEDs of the LED light source, and/or may suddenly drop in intensity during the dimming procedure (e.g., from approximately 1% to off). For instance, when dimming an LED light source using the PWM technique, the on time of the duty cycle of the pulsed signal may reach a threshold where, if reduced any further, causes the LED light source to become instable or exhibit undesirable characteristics. Similarly, when dimming an LED light source using the CCR technique, the DC magnitude of the current provided to the LED light source may reach a threshold where, if reduced any further, causes the LED light source to become instable or exhibit undesirable characteristics.

As described herein, a load control device for controlling (e.g., dimming) an intensity of a lighting load to a low intensity level and/or off is provided. The load control device may comprise a power converter circuit, a load regulation circuit, and/or a control circuit. The power converter circuit may be operable to receive a rectified AC voltage and to generate a DC bus voltage. The load regulation circuit may be operable to receive the DC bus voltage and to control a magnitude of a load current conducted through the lighting load, for example, using the DC bus voltage. The control circuit may be operatively coupled to the load regulation circuit for pulse width modulating and/or pulse frequency modulating the load current to control the intensity of the lighting load to a target intensity. The lighting load may comprise an LED light source. The load regulation circuit may comprise an LED drive circuit.

The control circuit may be configured to control the intensity of the lighting load by pulse width modulating the load current when the target intensity is above a predetermined threshold and control the intensity of the lighting load by pulse frequency modulating the load current when the target intensity is below the predetermined threshold. The predetermined threshold may be, for example, a low-end intensity (e.g., 1%). Pulse width modulating the load current may comprise maintaining a frequency of the load current constant and adjusting an on time of the load current. Pulse frequency modulating the load current may comprise maintaining the on time of the load current constant and adjusting the frequency of the load current.

For example, the control circuit may be configured to maintain the frequency of the load current at a normal pulse width modulation (PWM) frequency and adjust the on time of the load current between a maximum on time and a minimum on time when the target intensity is above the predetermined threshold, for example, when the target intensity is between a high-end intensity and the low-end intensity. The control circuit may be configured to maintain the on time of the load current at the minimum on time and adjust the frequency of the load current between the normal PWM frequency and a minimum PWM frequency when the target intensity is below the predetermined threshold, for example, when the target intensity is between the low-end intensity and a minimum intensity. The minimum intensity may be below (i.e., less than) the low-end intensity. The control circuit may be configured to maintain the frequency of the load current at the minimum PWM frequency and adjust the on time of the load current between the minimum on time and an ultra-low minimum on time when the target intensity is below the minimum intensity, for example, when the target intensity is between the minimum intensity and an ultra-low minimum intensity. For instance, the control circuit may dim the LED light source to off (i.e., the ultra-low minimum intensity may be 0% intensity).

The control circuit may be configured to dim the LED light source to off. For example, the control circuit may be configured to pulse width modulate the load current when the target intensity is below the minimum intensity, which is below the predetermined threshold (e.g., a low-end intensity). As such, the control circuit may be configured to control the intensity of the lighting load from the minimum intensity to off by pulse width modulating the load current. The control circuit may be configured to control the intensity of the lighting load from the predetermined threshold to off by pulse frequency modulating the load current. The control circuit may be configured to maintain a frequency of the load current constant, maintain an on time of the load current constant, and decrease a magnitude of the DC bus voltage when the target intensity is below the minimum intensity. For example, control circuit may control the intensity of the lighting load to off by decreasing the magnitude of the DC bus voltage.

The control circuit may be configured to control the intensity of the lighting load by pulse width modulating the load current when the target intensity is within a first intensity range and control the intensity of the lighting load by pulse frequency modulating the load current when the target intensity is within a second intensity range. The first intensity range may be greater than or less than the second intensity range. The control circuit may be configured to receive a command and control (e.g., dim) the intensity of the lighting load below the first intensity range and below the second intensity range to off. For example, the load control circuit may be configured to control the intensity of the lighting load below the second intensity range to off by pulse width modulating and/or pulse frequency modulating the load current. The load control circuit may be configured to control the intensity of the lighting load below the first intensity range and below the second intensity range to off by maintaining the frequency of the load current constant, maintaining the on time of the load current constant, and decreasing the magnitude of the DC bus voltage.

is a block diagram of an example system that comprises a light-emitting diode (LED) driver for controlling the intensity of an LED light source. A systemmay comprise an alternating-current (AC) power source, a dimmer switch, an LED driver, and/or an LED light source. The LED drivermay control an intensity of the LED light source. An example of the LED light sourcemay be an LED light engine. The LED light sourceis shown as a plurality of LEDs connected in series but may comprise a single LED or a plurality of LEDs connected in series, parallel, or a suitable combination thereof, for example, depending on the particular lighting system. The LED light sourcemay comprise one or more organic light-emitting diodes (OLEDs).

The LED drivermay be coupled to the AC power sourcevia the dimmer switch. The dimmer switchmay generate a phase-control signal V(e.g., a dimmed-hot voltage). The dimmer switchmay provide the phase-control signal Vto the LED driver. The dimmer switchmay comprise a bidirectional semiconductor switch (not shown), such as, for example, a triac or two anti-series-connected field-effect transistors (FETs), which may be coupled in series between the AC power sourceand the LED driver. The dimmer switchmay control the bidirectional semiconductor switch to be conductive for a conduction period Teach half-cycle of the AC power sourceto generate the phase-control signal V.

The LED drivermay turn the LED light sourceon and off in response to the conduction period Tof the phase-control signal Vreceived from the dimmer switch. The LED drivermay adjust (i.e., dim) a present intensity Lof the LED light sourceto a target intensity Lin response to the phase-control signal V. The target intensity Lmay range across a dimming range of the LED light source. For example, the dimming range of the LED light sourcemay be between a low-end intensity L(e.g., approximately 1%) and a high-end intensity L(e.g., approximately 100%). The LED drivermay control the magnitude of a load current Ithrough the LED light sourceand/or the magnitude of a load voltage Vacross the LED light source. Accordingly, the LED drivermay control at least one of the load voltage Vacross the LED light sourceand the load current Ithrough the LED light source to control the amount of power delivered to the LED light source, for example, depending upon a mode of operation of the LED driver (e.g., as described herein).

The LED drivermay work with (i.e., control) a plurality of different LED light sources. For example, the LED drivermay work with LED lights sources that are rated to operate using different load control techniques, different dimming techniques, and/or different magnitudes of load current and/or voltage. The LED drivermay control the magnitude of the load current Ithrough the LED light sourceand/or the load voltage Vacross the LED light source using different modes of operation. For example, the LED drivermay use a current load control mode (i.e., for using the current load control technique) and/or a voltage load control mode (i.e., for using the voltage load control technique). The LED drivermay adjust the magnitude to which the LED drivercontrols the load current Ithrough the LED light sourcein the current load control mode. The LED drivermay adjust the magnitude to which the LED drivercontrols the load voltage Vacross the LED light source in the voltage load control mode.

When operating in the current load control mode, the LED drivermay control the intensity of the LED light sourceusing a PWM dimming mode (i.e., for using the PWM dimming technique), a CCR dimming mode (i.e., for using the CCR dimming technique), and/or a pulse frequency modulation (PFM) dimming mode (i.e., for using the PFM dimming technique). In the PWM dimming mode, the LED drivermay control the load current Iby altering the pulse duration of the load current Iand maintaining the frequency of the load current Iconstant. In the PFM dimming mode, the LED drivermay control the load current Iby maintaining the pulse duration of the load current Iconstant and altering the frequency of the load current I. In the CCR dimming mode, the LED drivermay control the load current Iby altering the DC magnitude of the current load current I. When operating in the voltage load control mode, the LED drivermay control the amount of power delivered to the LED light sourceusing the PWM dimming mode and/or the PFM dimming mode. The LED drivermay control the amount of power delivered to the LED light sourcein response to a digital message, which may be received from a communication circuit, for example as described herein.

is a block diagram of an example of an LED driver for controlling an LED light source. An LED drivermay comprise a radio-frequency (RFI) filter and rectifier circuit, a buck-boost flyback converter, a bus capacitor CBUS, an LED drive circuit, a control circuit, a power supply, a phase-control input circuit, memory, and/or a communication circuit. The LED drivermay be an example of the LED driverof. As such, the LED drivermay be used within the systemof. The LED drivermay control an LED light source, such as the LED light source.

The RFI filter and rectifier circuitmay receive the phase-control signal Vfrom a dimmer switch (e.g., the dimmer switchof). The RFI filter and rectifier circuitmay minimize the noise provided on an AC power source (e.g., the AC power sourceof). The RFI filter and rectifier circuitmay generate a rectified voltage V. The buck-boost flyback convertermay receive the rectified voltage V. The buck-boost flyback convertermay generate a variable direct-current (DC) bus voltage Vacross the bus capacitor C. The buck-boost flyback convertermay provide electrical isolation between the AC power source and the LED light source. The buck-boost flyback convertermay operate as a power factor correction (PFC) circuit to adjust the power factor of the LED drivertowards a power factor of one. The buck-boost flyback convertermay be a power converter circuit. Although illustrated as the buck-boost flyback converter, the LED drivermay comprise any suitable power converter circuit for generating an appropriate bus voltage V, such as, for example, a boost converter, a buck converter, a single-ended primary-inductor converter (SEPIC), a Ćuk converter, or other suitable power converter circuit. The bus voltage Vmay be characterized by some voltage ripple as the bus capacitor Cperiodically charges and discharges.

The LED drive circuitmay be a load regulation circuit. The LED drive circuitmay receive the bus voltage V. The LED drive circuitmay control the amount of power delivered to the LED light sourceso as to control the intensity of the LED light source. The LED drive circuitmay comprise a controllable-impedance circuit, such as a linear regulator, for example, as described herein. The LED drive circuitmay comprise a switching regulator, such as a buck converter for example. Examples of various embodiments of LED drive circuitsare described in U.S. patent application Ser. No. 12/813,908, filed Jun. 11, 2010, entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE, the entire disclosure of which is hereby incorporated by reference.

The control circuitmay control the operation of the buck-boost flyback converterand/or the LED drive circuit. The control circuitmay comprise, for example, a controller or any other suitable processing device, such as, for example, a microcontroller, a programmable logic device (PLD), a microprocessor, an application specific integrated circuit (ASIC), or a field-programmable gate array (FPGA). The power supplymay receive the rectified voltage V. The power supplymay generate a plurality of direct-current (DC) supply voltages for powering the circuitry of the LED driver, for example, using the rectified voltage V. For example, the power supplymay generate a first non-isolated supply voltage V(e.g., approximately 14 volts) for powering the control circuitry of the buck-boost flyback converter, a second isolated supply voltage V(e.g., approximately 9 volts) for powering the control circuitry of the LED drive circuit, and/or a third non-isolated supply voltage V(e.g., approximately 5 volts) for powering the control circuit.

The control circuitmay be coupled to the phase-control input circuit. The phase-control input circuitmay generate a target intensity control signal V. The target intensity control signal Vmay comprise, for example, a square-wave signal having a duty cycle DC, which may be dependent upon the conduction period Tof the phase-control signal Vreceived from a dimmer switch (e.g., the dimmer switchof). The duty cycle DCmay be representative of the target intensity Lof the LED light source. The target intensity control signal Vmay comprise a DC voltage having a magnitude dependent upon the conduction period Tof the phase-control signal V, and thus representative of the target intensity Lof the LED light source.

The control circuitmay be coupled to the memory. The memorymay store the operational characteristics of the LED driver(e.g., the load control mode, the dimming mode, the magnitude of the rated load voltage or current, and/or the like). The communication circuitmay be coupled to, for example, a wired communication link or a wireless communication link, such as a radio-frequency (RF) communication link or an infrared (IR) communication link. The control circuitmay update the target intensity Lof the LED light sourceand/or the operational characteristics stored in the memoryin response to digital messages received via the communication circuit. For example, the LED drivermay receive a full conduction AC waveform from the AC power source (i.e., not the phase-control signal Vfrom the dimmer switch) and may determine the target intensity Lfor the LED light sourcefrom the digital messages received via the communication circuit.

The control circuitmay manage the operation of the buck-boost flyback converterand/or the LED drive circuitto control the intensity of the LED light source. The control circuitmay receive a bus voltage feedback signal V, which may be representative of the magnitude of the bus voltage V, from the buck-boost flyback converter. The control circuitmay provide a bus voltage control signal Vto the buck-boost flyback converterfor controlling the magnitude of the bus voltage Vto a target bus voltage V(e.g., from approximately 8 volts to 60 volts). The LED drive circuitmay control a peak magnitude Iof the load current Iconducted through the LED light sourcebetween a minimum load current Iand a maximum load current I(e.g., when operating in the current load control mode), for example, in response to a peak current control signal Vprovided by the control circuit. The control circuitmay receive a load current feedback signal V, which is representative of an average magnitude Iof the load current Iflowing through the LED light source. The control circuitmay receive a regulator voltage feedback signal V, which is representative of the magnitude of a regulator voltage V(i.e., a controllable-impedance voltage) across the linear regulator of the LED drive circuit, for example, as described herein.

The control circuitmay control the LED drive circuitto control the amount of power delivered to the LED light sourceusing the current load control mode of operation and/or the voltage load control mode of operation. During the current load control mode, the LED drive circuitmay regulate the peak magnitude Iof the load current Ithrough the LED light sourceto control the average magnitude Ito a target load current Iin response to the load current feedback signal V(i.e., using closed loop control). The target load current Imay be stored in the memory. The target load current Imay be programmed to be any specific magnitude depending upon the LED light source.

To control the intensity of the LED light sourceduring the current load control mode, the control circuitmay control the LED drive circuitto adjust the amount of power delivered to the LED light sourceusing the PWM dimming technique, the PFM dimming technique, and/or the CCR dimming technique. Using the PWM dimming technique, the control circuitmay control the peak magnitude Iof the load current Ithrough the LED light sourceto the target load current I. Using the PWM dimming technique, the control circuitmay pulse-width modulate the load current Ito dim the LED light sourceand achieve the target load current I. For example, the LED drive circuitmay control (i.e., adjust) a duty cycle DCof the load current Iin response to a duty cycle DCof a dimming control signal Vprovided by the control circuit. Further, when using the PWM dimming technique, the LED drive circuitmay maintain a frequency fof the load current Iin response to a frequency fof the dimming control signal Vprovided by the control circuit. The intensity of the LED light sourcemay be dependent upon the duty cycle DCand the frequency fof the pulse-width modulated load current I.

Using the PFM dimming technique, the control circuitmay control the peak magnitude Iof the load current Ithrough the LED light sourceto the target load current I. Using the PFM dimming technique, the control circuitmay pulse frequency modulate the load current Ito dim the LED light sourceand achieve the target load current I. For example, the LED drive circuitmay control (i.e., adjust) a frequency fof the load current Iin response to a frequency fof a dimming control signal Vprovided by the control circuit. Further, when using the PFM dimming technique, the LED drive circuitmay maintain the duty cycle DCof the load current Iin response to a duty cycle DCof the dimming control signal Vprovided by the control circuit. The intensity of the LED light sourcemay be dependent upon the duty cycle DCand the frequency fof the pulse-width modulated load current I.

Using the CCR technique, the control circuitmay not pulse-width modulate or pulse-frequency modulate the load current I. Using the CCR technique, the control circuitmay adjust the magnitude of the target load current Iso as to adjust the average magnitude Iof the load current Ithrough the LED light source. The average magnitude Iof the load current Ithrough the LED light sourcemay be equal to the peak magnitude Iof the load current Iin the CCR dimming mode.

During the voltage load control mode, the LED drive circuitmay regulate the DC voltage of the load voltage Vacross the LED light sourceto a target load voltage V. The target load voltage Vmay be stored in the memory. The target load voltage Vmay be programmed to be any specific magnitude depending upon the LED light source. The control circuitmay dim the LED light sourceusing the PWM dimming technique and/or the PFM dimming technique during the voltage load control mode. For example, using the PWM dimming technique, the control circuitmay adjust a duty cycle DCof the load voltage Vin response to a duty cycle DCof the dimming control signal Vto dim the LED light source. Using the PFM dimming technique, the control circuitmay adjust the frequency fof the load voltage Vin response to a frequency fof the dimming control signal Vto dim the LED light source. An example of a configuration procedure for the LED driveris described in greater detail in U.S. patent application Ser. No. 12/813,989, filed Jun. 11, 2010, entitled CONFIGURABLE LOAD CONTROL DEVICE FOR LIGHT-EMITTING DIODE LIGHT SOURCES, the entire disclosure of which is hereby incorporated by reference.

is a schematic diagram of an example of a flyback converter and an LED drive circuit. A flyback convertermay comprise a flyback transformer, a field-effect transistor (FET) Q, a diode D, a resistor R, a resistor R, a flyback control circuit, a filter circuit, an optocoupler circuit, and/or a feedback resistor R. An LED drive circuitmay comprise a regulation field-effect transistor (FET) Q, a filter circuit, an amplifier circuit, a gate resistor R, a feedback circuit, a dimming FET Q, a sample and hold circuit (SHC), and/or an overvoltage protection circuit. The flyback convertermay be an example of the buck-boost flyback converterof. The LED drive circuitmay be an example of the LED drive circuitof. As such, the LED driverofand/or the LED driverofmay comprise the flyback converterand/or the LED drive circuit.

The flyback transformermay comprise a primary winding and a secondary winding. The primary winding may be coupled in series with the field-effect transistor (FET) Q. Although illustrated as the field-effect transistor (FET) Q, the primary winding of the flyback transformermay be coupled in series with any flyback switching transistor or other suitable semiconductor switch. The secondary winding of the flyback transformermay be coupled to the bus capacitor Cvia the diode D. The bus voltage feedback signal Vmay be generated by a voltage divider comprising the resistors R, Rcoupled across the bus capacitor C.

The flyback control circuitmay receive the bus voltage control signal Vfrom the control circuit, for example, via the filter circuitand the optocoupler circuit. The filter circuitand the optocoupler circuitmay provide electrical isolation between the flyback converterand the control circuit. The flyback control circuitmay comprise, for example, part number TDA4863, manufactured by Infineon Technologies. The filter circuitmay generate a filtered bus voltage control signal Vusing the bus voltage control signal V. For example, the filter circuitmay comprise a two-stage resistor-capacitor (RC) filter for generating the filtered bus voltage control signal V. The filtered bus voltage control signal Vmay comprise a DC magnitude dependent upon the duty cycle DCof the bus voltage control signal V. The flyback control circuitmay receive a control signal representative of the current through the FET Qfrom the feedback resistor R, which is coupled in series with the FET Q.

The flyback control circuitmay control the FET Qto selectively conduct current through the flyback transformerto generate the bus voltage V. The flyback control circuitmay render the FET Qconductive and non-conductive at a high frequency (e.g., approximately 150 kHz or less), for example, to control the magnitude of the bus voltage Vin response to the DC magnitude of the filtered bus voltage control signal Vand the magnitude of the current through the FET Q. For example, the control circuitmay increase the duty cycle DCof the bus voltage control signal Vsuch that the DC magnitude of the filter bus voltage control signal Vincreases in order to decrease the magnitude of the bus voltage V. The control circuitmay decrease the duty cycle DCof the bus voltage control signal Vto increase the magnitude of the bus voltage V. The filter circuitmay provide a digital-to-analog conversion for the control circuit(i.e., from the duty cycle DCof the bus voltage control signal Vto the DC magnitude of the filtered bus voltage control signal V). The control circuitmay comprise a digital-to-analog converter (DAC) for generating (e.g., directly generating) the bus voltage control signal Vhaving an appropriate DC magnitude for controlling the magnitude of the bus voltage V.

is a schematic diagram of an example of the LED drive circuit of. The LED drive circuitmay comprise the regulation field-effect transistor (FET) Q, the filter circuit, the amplifier circuit, the gate resistor R, the feedback circuit, the dimming FET Q, the sample and hold circuit, and/or the overvoltage protection circuit. The feedback circuitmay comprise a feedback resistor R, a filter circuit, and/or an amplifier circuit. The sample and hold circuitmay comprise a FET Q, a capacitor C, a resistor R, a resistor R, a FET Q, a resistor R, and/or a resistor R. The overvoltage protection circuitmay comprise a comparator U, a resistor R, a resistor R, a resistor R, a resistor R, a filtering capacitor C, a resistor R, and/or a resistor R.

The LED drive circuitmay comprise a linear regulator (i.e., a controllable-impedance circuit) including the regulation field-effect transistor (FET) Qcoupled in series with the LED light sourcefor conducting the load current I. Although illustrated as the FET Q, the LED drive circuitmay comprise any power semiconductor switch coupled in series with the LED light sourcefor conducting the load current I. The regulation FET Qmay comprise a bipolar junction transistor (BJT), an insulated-gate bipolar transistor (IGBT), or any suitable transistor. The peak current control signal Vprovided by the control circuitmay be coupled to the gate of the regulation FET Qthrough the filter circuit, the amplifier circuit, and the gate resistor R. The control circuitmay control the duty cycle DCof the peak current control signal Vto control the peak magnitude Iof the load current Iconducted through the LED light sourceto the target load current I.

The filter circuit(e.g., a two-stage RC filter) may provide digital-to-analog conversion for the control circuit, for example, by generating a filtered peak current control signal V. The filtered peak current control signal Vmay have a DC magnitude dependent upon the duty cycle DCof the peak current control signal Vand may be representative of the magnitude of the target load current I. The control circuitmay comprise a DAC for generating (e.g., directly generating) the peak current control signal Vhaving an appropriate DC magnitude for controlling the peak magnitude Iof the load current I. The amplifier circuitmay generate an amplified peak current control signal V. The amplifier circuitmay provide the amplified peak current control signal Vto the gate of the regulation transistor Qthrough the resistor R, such that a drive signal at the gate of the regulation transistor Q, e.g., a gate voltage V, has a magnitude dependent upon the target load current I. The amplifier circuitmay comprise a standard non-inverting operational amplifier circuit having, for example, a gain α of approximately three.

The feedback resistor Rof the feedback circuitmay be coupled in series with the regulation FET Q, for example, such that the voltage generated across the feedback resistor is representative of the magnitude of the load current I. For example, the feedback resistor Rmay have a resistance of approximately 0.0375Ω. The filter circuit(e.g., a two-stage RC filter) of the feedback circuitmay be coupled between the feedback resistor Rand the amplifier circuit(e.g., a non-inverting operational amplifier circuit having a gain β of approximately 20). The amplifier circuitmay have a variable gain, which for example, may be controlled by the control circuitand could range between approximately 1 and 1000. The amplifier circuitmay generate the load current feedback signal V. The amplifier circuitmay provide the load current feedback signal Vto the control circuit. The load current feedback signal Vmay be representative of an average magnitude Iof the load current I, e.g.,

wherein Ris the resistance of the feedback resistor R. Examples of other feedback circuits for the LED drive circuitare described in greater detail in U.S. patent application Ser. No. 12/814,026, filed Jun. 11, 2010, entitled CLOSED-LOOP LOAD CONTROL CIRCUIT HAVING A WIDE OUTPUT RANGE, the entire disclosure of which is hereby incorporated by reference.

When operating in the current load control mode, the control circuitmay control the regulation FET Qto operate in the linear region, such that the peak magnitude Iof the load current Iis dependent upon the DC magnitude of the gate voltage Vat the gate of the regulation FET Q. In other words, the regulation FET Qmay provide a controllable-impedance in series with the LED light source. If the magnitude of the regulator voltage Vdrops too low, the regulation FET Qmay be driven into the saturation region, such that the regulation FET Qbecomes fully conductive and the control circuitis no longer able to control the peak magnitude Iof the load current I. Therefore, the control circuitmay adjust the magnitude of the bus voltage Vto prevent the magnitude of the regulator voltage Vfrom dropping below a minimum regulator voltage threshold V(e.g., approximately 0.4 volts). In addition, the control circuitmay adjust the magnitude of the bus voltage Vto control the magnitude of the regulator voltage Vto be less a maximum regulator voltage threshold V(e.g., approximately 0.6 volts), for example, to prevent the power dissipated in regulation FET Qfrom becoming too large, thus increasing the total efficiency of the LED driver (e.g., the LED driver, the LED driver, and/or the like). Since the regulator voltage Vmay have some ripple (e.g., which may be due to the ripple of the bus voltage V), the control circuitmay determine the minimum value of the regulator voltage Vduring a period of time and to compare this minimum value of the regulator voltage Vto the regulator voltage threshold Vand the maximum regulator voltage threshold V.

When operating in the voltage load control mode, the control circuitmay drive the regulation FET Qinto the saturation region, for example, such that the magnitude of the load voltage Vis approximately equal to the magnitude of the bus voltage V(e.g., minus the small voltage drops due to the on-state drain-source resistance Rof the FET regulation Qand the resistance of the feedback resistor R).

The dimming FET Qof the LED drive circuitmay be coupled between the gate of the regulation FET Qand circuit common. The dimming control signal Vfrom the control circuitmay be provided to the gate of the dimming FET Q. When the dimming FET Qis rendered conductive, the regulation FET Qmay be rendered non-conductive. When the dimming FET Qis rendered non-conductive, the regulation FET Qmay be rendered conductive.

While using the PWM dimming technique during the current load control mode, the control circuitmay adjust the duty cycle DCof the dimming control signal V(e.g., to adjust the length of an on time tthat the regulation FET Qis conductive) to control when the regulation FET Qconducts the load current Iand to control the intensity of the LED light source. For example, the control circuitmay generate the dimming control signal Vusing a constant frequency f(e.g., approximately in the range of 500-550 Hz), such that the on time tof the dimming control signal Vis dependent upon the duty cycle DC, i.e.,

As the duty cycle DCof the dimming control signal Vincreases, the duty cycle DC, DCof the corresponding load current Ior load voltage Vdecreases, and vice versa.

While using the PFM dimming technique during the current load control mode, the control circuitmay adjust the frequency fof the dimming control signal Vto control the frequency at which the regulation FET Qconducts the load current Iand to control the intensity of the LED light source. For example, the control circuitmay generate the dimming control signal Vusing a constant on time t, such that the frequency fof the dimming control signal Vis dependent upon the duty cycle DC, i.e.,

As the duty cycle DCof the dimming control signal Vincreases, the duty cycle DC, DCof the corresponding load current Ior load voltage Vdecreases, and vice versa.

When using the PWM dimming technique and/or the PFM dimming technique in the current load control mode, the control circuitmay control the peak magnitude Iof the load current Iin response to the load current feedback signal Vto maintain the average magnitude Iof the load current Iconstant (i.e., at the target lamp current I). The control circuitmay calculate the peak magnitude Iof the load current Ifrom the load current feedback signal Vand the duty cycle DCof the dimming control signal V, i.e.,

The load current feedback signal Vmay be representative of the average magnitude Iof the load current I. When using the CCR dimming technique during the current load control mode, the control circuitmay maintain the duty cycle DCof the dimming control signal Vat a high-end dimming duty cycle DC(e.g., approximately 0%, such that the FET Qis always conductive) and/or may adjust the target load current I(e.g., via the duty cycle DCof the peak current control signal V) to control the intensity of the LED light source.